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    There are four fundamental instruments necessary for proper luminous tube processing using the internal bombarding method. Each has its own specific purpose and function and one is equally as important as the other. Without the full compliment of these instruments to inform the processing technician of all aspects of the processing procedure, the process will be little more than guesswork as to the quality of the finished product.

    These four instruments are:

    1. A gas fill pressure gauge (also used to monitor tube pressure during bombarding).
    2. A high vacuum gauge to determine ultimate vacuum and system integrity.
    3. An A.C. milliamperes meter (mA. meter) to measure bombarder current.
    4. A temperature gauge to measure glass temperature.

    Some equipment suppliers offer certain types of these instruments to the industry that are neither a good choice nor practical for luminous tube processing when using the high voltage internal bombarding method. They can even hinder the processing technician rather than help them. However, it is important to point out that although SVP sells what we feel is the best choice of instruments for our purposes and promotes them on this website, we are not the only company they are available from*. This is not an endorsement for something only we sell, with the exception of the SVP Bombarding Temperature Gauge. Similar products that will work well for our purposes are available from other manufacturers. Therefore, the following information is presented to inform interested readers rather than persuade them and is done so in an effort to improve the overall quality of neon throughout our industry. Following is a short discourse for each instrument that gives a brief summary of what the instrument is used for and covers key points to consider when selecting a specific instrument.

    * The SVP Bombarding Temperature Gauge is the only instrument of it’s kind currently on the market and is specifically made for high voltage bombarding purposes.

    GAS FILL PRESSURE GAUGE (back-fill gauge, filling gauge)

    VACUUM GAUGE (micron gauge, ultimate vacuum gauge, high vacuum gauge)
    BOMBARDING MILLIAMPERES METER (milliammeter, mA meter, current meter)
    BOMBARDING TEMPERATURE GAUGE (heat gauge, pyrometer)


    This instrument is used to measure how much rare gas is put in the unit (the backfill pressure) after processing is complete. Because these instruments are absolute pressure gauges they are also commonly used to monitor tube pressure during the bombarding procedure.

    There are several instruments currently on the market used for measuring the amount of gas fill pressure; Digital instruments with a simulated analog readout, older types of digital instruments with a numerical readout, an analog capsule dial gauge (typically referred to as a “Torr gauge”), oil manometers (typically referred to as a U-gauge or butyl gauge), as well as other less common instruments. The most popular and user friendly is the capsule dial gauge (Torr gauge). However, if the technician is concerned with accuracy and repeatability from unit to unit, this is not the best choice for several reasons.

    Digital and dial gauges generally have an accuracy shift of about ± 2% of full scale. For the popular 0-40 Torr gauge it is in fact ± 2% of full scale. This is ± 0.8 mm (2% of 40). This means that the actual fill pressure, regardless of what the gauge is indicating, may vary by as much as 1.6 mm from one unit to the next (from -0.8 to +0.8 of the reading). When filling a 15 mm tube to the desired pressure of 9 mm, the actual fill pressure can be off by as much as 18% from one unit to the next. For a digital gauge the value of the accuracy shift depends on the range of the instrument, but typically they are 0-760 Torr (mm) full scale. When considering a possible accuracy shift of ± 2% of full scale for a 0-760 range the results are ridiculously poor at best and should be completely unacceptable to a technician who is concerned with producing quality, trouble free neon. This much deviation from the correct fill pressure can adversely affect a number of things; Overall tube operation, transformer load and the service life of each.

    These already problematic gauges reveal other shortcomings as well. Both exhibit virtual leaks*. The 0-40 Torr gauge (or equivalent; several renditions are available) is of particular concern, so much so that the systems ultimate vacuum may never be reached even after days of continuous pumping. Depending on the particular method used to connect the gauge to the manifold, digital gauges can show similar problems. Even if the ultimate vacuum is reached due to the pumps capacity and ability to do so, closing the main vacuum stopcock will immediately defeat that achievement. Further, the Torr gauge has a very delicate movement. Even small debris such as phosphor powder will adversely affect the movement if allowed to infiltrate it – something that is easily accomplished through the normal course of processing. Larger particulates such as glass fragments that may inadvertently get into the movement due to a mishap during processing can damage the movement beyond repair. Once the movement is contaminated and compromised it does not move freely or accurately. Common observations are that of the needle sticking, not going to 0 when a hard vacuum is in the manifold, greater accuracy shifts than those previously mentioned, or all of these combined.

    Although archaic by some opinions, the U-gauge oil manometer is by far the most accurate, repeatable and dependable instrument for our purposes. In comparison to other instruments the U-gauge is as accurate as how well you can see the scale**. Filling units to ± 0.1 Torr (mm) is entirely possible and repeatable from one unit to the next. There is no accuracy shift from one unit to the next; what you see is what you get. Also unlike other instruments, the only recalibration it ever needs is to periodically be cleaned and refilled with new oil of the appropriate type. The neon technician can do this in-house, thereby eliminating the need to return it to the factory for recalibration, as is the case with other fill gauges. There are no virtual leaks, delicate movements or circuit boards to be damaged.

    * Virtual leaks are sources of gases (air, water vapor, neon & argon gas from back-filling, etc.) that are partially trapped but are released at a slow rate into the system. For example, un-vented screw holes or other threads exposed to the interior of the vacuum system will release gases into the system over a long period of time and make it impossible to reach and maintain ultimate vacuum until the partially trapped gases are completely exhausted. This can take days and even weeks. If the virtual leak is then again exposed to atmospheric pressure, or even back-fill pressures, it will be replenished and like starting over to remove the gases. Therefore, it is important to eliminate sources of virtual leaks from the system.

    ** The scale must be correctly calibrated for the particular fluid used and the gauge constructed correctly for the corresponding scale. Using butyl phthalate oil with a scale calibrated for silicone oil, or vice-versa, will give incorrect pressure readings. Similarly, a scale calibrated for a standard U-gauge cannot be used with the new advanced, compact SVP U-Gauge Fluid Manometer as there is a multiplied pressure differential between the two columns. Therefore, the scale for the new SVP Manometer is specific to this instrument when used with the oil supplied and no other gauge or oil.


    A high vacuum gauge is used to measure the ultimate vacuum obtained within the manifold following the processing procedure and prior to backfilling the tube with inert gas. This instrument ensures that the tube was adequately evacuated. A high vacuum gauge is also valuable in determining the integrity of the vacuum system, as well as troubleshooting problems if they arise. Once a technician has become familiar with this instrument and how it reacts to various conditions and situations it can also be invaluable in avoiding problems.

    A vacuum gauge capable of measuring down to at least 1 micron should be used and this level of vacuum should be strived for. A vacuum of 1 micron is where molecular flow begins to take place and where the high vacuum region begins. Analog vacuum gauges, rather than digital vacuum gauges, are best suited for our purposes for various reasons. The high voltage field produced by the bombarder is generally not favorable toward digital instrumentation (both vacuum gauges and temperature gauges). In addition to the influence of high voltage, the inexpensive (regardless of what they are sold for) digital vacuum gauges that are typically offered to the U.S. neon industry are not true high vacuum gauges, even though they are referred to as such by the suppliers offering them. Some sell these instruments with a gauge tube (sensor) that is designed for a range of measurement of 0-1,000 microns (the yellow coded Hastings DV-6M gauge tube) in an attempt to improve the accuracy of the unit. However, the gauge itself is actually designed for use with a 0-20,000 micron gauge tube. 20,000 microns = 20 Torr (mm), so this gauge is actually a 0-20 mm pressure gauge, not a high vacuum gauge intended to measure in the low micron range. A vacuum gauge designed to measure millimeters of pressure rather than pressures in the low micron range cannot accurately measure a few microns of pressure, much less measure 1 micron or below, regardless of what gauge tube is supplied with it and regardless of what the supplier claims. Further, repeatability of a gauge such as this in the low micron range is poor at best. This can lead the processing technician to think there is a problem with the vacuum system when there is not, or worse yet, think the vacuum is better than it actually is.

    When considering a vacuum gauge, one that will measure pressures between 1 micron and 1,000 microns is suitable for general neon tube production. However, one that can measure 0.1 micron should be used for critical neon work and cold cathode lamp production. Generally speaking, the broader the measuring range that the instrument covers the less accurate it will be in the low micron range, i.e., a vacuum gauge that has a scale of 0-1,000 microns will not measure a vacuum of 1 micron as accurately as a gauge that has a scale of 0-100 microns. A comparison of these two analog vacuum gauge ranges can be seen Here. For reference, a vacuum chart with pressure conversions is available Here.

    Whatever vacuum gauge is used, whether it is line voltage operated or battery operated, a stopcock between the vacuum gauge tube (sensor) and main manifold body must be used to isolate the gauge tube from the main manifold. This protects the gauge and gauge tube from possible damage due to bombarder high voltage, spark tester frequencies and voltage, as well as the influx of contaminants, which are a normal result of the bombarding process and venting the manifold to atmosphere. The optional SVP Vacuum Gauge Stopcock and how the gauge tube is connected to the stopcock can be seen Here.


    A bombarding milliamperes meter (mA meter) monitors the amount of current generated by the bombarder through the tube being processed. The use of this instrument is necessary to ensure that the right amount of current is being applied at the appropriate times. This eliminates any guesswork and provides the processing technician with the information necessary to avoid the problems normally associated with tube processing if this instrument is not used. Damage to the phosphor coating inside the tube, electrode sputtering and structural damage to the glass tube will result if the bombarding current is not monitored with an appropriate milliamperes meter.

    When considering an A.C. milliamperes meter to measure bombarding current, one that has a range of 0-1,000 mA. is suitable for general neon work as the maximum current applied will typically be less than 1,000 mA. For larger diameter, 25mm Cold Cathode work a 0-2,000 mA. meter should be used due to the higher currents required to process the larger electrodes. Bombarding current in this instance will easily exceed 1,000 mA.

    A True RMS Iron-Vane meter should be used rather than an inexpensive rectified meter. A rectified A.C. milliamperes meter, which gives average values rather than actual values, is not accurate enough or suitable when used in close proximity to the high voltage field produced by the bombarding transformer. Further, certain types of bombarder choke controls, if not closely matched to the bombarder, distort and/or chop the sine wave and create excessive signal noise. The more distorted the waveform is and the more signal noise there is the more inaccurate the reading will be on a rectified meter. This accuracy shift can be as much as 20% from the actual value. For example, a reading of 600 mA on a rectified meter may actually be anywhere from 480 mA to 720 mA. By comparison, a good quality true RMS Iron-Vane meter is unaffected by the applied voltage, wave distortion or signal noise and will be within ± 2% of the actual value. Each type of meter is easily identified visually. A comparison of the two different meter types and how to identify each can be seen Here.


    A Bombarding Temperature Gauge* monitors the glass temperature during the processing procedure. A minimum glass temperature is necessary to ensure that the maximum amount of contaminants and impurities are released from the internal surface of the tubing, which if allowed to remain, will affect the life, efficiency and overall quality of the finished unit. Too high of a glass temperature is also undesirable. Excessive glass temperature will damage phosphor coatings, cause deformation of the glass structure and induce stress points into the glasswork. This instrument, if properly designed for this specific purpose, eliminates speculation as to the actual glass temperature and provides the technician with reliable information. In the case of the SVP Bombarding Temperature Gauge, the instrument is also used to correlate glass cool-down temperatures with vacuum levels obtained as indicated by the vacuum gauge.

    Because of the high voltage field, consideration must be given when choosing a bombarding temperature gauge. Inexpensive infrared pyrometers and inexpensive digital gauges (regardless of what they are sold for) as well as thermal mediums such as temperature crayons** are not suitable for various reasons and are found to be inaccurate in close proximity to the bombarder high voltage field. However, good quality analog type pyrometers are historically the best choice as they are typically unaffected by the high voltage field.

    When used in close proximity to high voltage an infrared pyrometer requires special circuitry. If the lens, or signal pick-up or head, is to be placed close to the tube being bombarded a special lens and focal point must also be used for the gauge to function properly. Such instruments are available, but the design requirements and construction criteria add considerable cost to the finished product. Because of the cost factor, infrared temperature gauges suitable for bombarding are rarely used. However, they are available.

    The digital temperature gauges most commonly offered to the neon industry at the present time are also affected by the bombarder high voltage electromagnetic field. The printed circuit board and related components that comprise these instruments were simply not designed for this. The result is an instrument that can display various readings at any given temperature depending on different factors including the load on the bombarder, which changes the characteristics of the emitted high voltage field. General observations reported have also been temperature readouts that go “haywire” once the gauge passes a certain temperature: i.e. The numbers begin to rapidly bounce around both upscale and downscale, or the numbers, which are comprised of LCD’s, “scramble” and are illegible. The manufacturer of these gauges is aware of these inherent problems. Their solution is to wrap the circuit board in aluminum foil in an attempt to shield it from the high voltage field.

    Regardless of which instrument and/or method are used to measure glass temperature, it should have the ability to easily compare glass cool-down temperatures with vacuum levels obtained during the evacuation stage. This is an important consideration in determining how well the unit was evacuated and therefore the quality of the finished product. For example, at a glass cool-down temperature of 175°C (the temperature at which vaporized contaminants will begin to re-condense inside the tube) the vacuum must be better than 5µ, preferably better. Obviously any type of thermal medium cannot do this. Only an instrument that provides temperature readout can.

    SVP Neon Equipment is the only equipment manufacturer (or materials supplier) who specifies evacuation speeds vs. glass cool-down temperatures. Refer to our Recommended Bombarding Procedure, also listed on the TECHNICAL page for more detailed information.

    SVP Neon Equipment is proud to be the only manufacturer and supplier of a Bombarding Temperature Gauge that is truly made specifically for high voltage bombarding purposes. With a large 4½” easy to read analog meter face especially designed to aid the processing technician with at-a-glance information, both during the heating stage as well as the cool-down evacuation stage, it is the most user-friendly and accurate Bombarding Temperature Gauge on the market.

    * Paper, regardless of which kind, should not be considered as an alternative or substitution. A very convincing argument against its use is to place a ½” wide strip of paper on a 75 watt light bulb for 10 minutes. Try several kinds of paper for comparison. The light bulb never changes temperature and the temperature is far less than bombarding temperature, but the paper will progressively get darker the longer it stays on the bulb. Different papers will char at different time intervals, but they will all turn brown in much the same manner as they do when using them to “measure” glass temperature. The amount of moisture in the paper and ambient atmospheric conditions will also affect the time required to char the paper. Attempting to use paper to measure glass temperature during bombarding, where a variety of conditions can exist, is an old unreliable method of doing so.

    ** Thermal mediums such as temperature crayons are compounds that rely on a series of chemical reactions to achieve the desired result. Their accuracy depends on the absence of anything other than heat that may influence the reaction, thereby changing the result. The influence of the high voltage field is why temperature crayons are not accurate for bombarding purposes because of the very nature of the medium. As the crayon is being heated it is going through chemical changes. First it liquefies, then begins to change colors, during which time it solidifies again in a different form. These changes are chemical reactions taking place. According to both the Department of Chemistry and Chemical Engineering Department at the University of South Carolina, whenever a voltage is introduced into a chemical reaction such as this, it changes the end result of the reaction. In the instance of high voltage, the more voltage that is present the more the end result of the chemical reaction will be changed. In our case the high voltage is more than sufficient to cause the crayon to change colors before it is supposed to. In other words, the crayon may say “300°C” on the label, but it will actually change color at a much lower temperature than this. To reflect on this situation, a supplier of temperature crayons to the neon industry has changed which temperature-range crayon they offer at least 3 or 4 times since they first started promoting them. The replacements have progressively been of a higher temperature rating; 260°C, 280°C, etc. With the latest offering the crayon labels have even been removed to hide what temperature rating the crayon is supposed to be. It is our opinion that the temperature designation of the crayon was changed to compensate for the affects mentioned above in an effort to get the crayon to change colors at an actual glass temperature closer to what it should be, rather than what the crayon is specified to change color at. However, the effort still seems to fall short of the target temperature. SVP has tested different renditions of these crayons over the years during tube processing. The older “green” crayon that was once used was marked as “300°C”. During high voltage processing this crayon would turn to almost black at ~180°C and stop “smoking” at an actual glass temperature of ~225°C. The new tan/brown crayon with no label will turn to the required “chestnut” color at ~175°C and stopped smoking at ~200°C – far short of the required glass temperature. However, when a large amount of the tan crayon was applied it increased the color change temperature to ~200°C and the “stopped smoking” temperature to ~250°C – hardly a reliable, repeatable, consistent method for measuring glass temperature during high voltage bombarding.



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    Silica Vacuum Products